THE COSTS AND BENEFITS OF BASAL INFECTION RESISTANCE VS DIVERSE IMMUNE PRIMING RESPONSES IN AN INSECT

ABSTRACT In insects, basal pathogen resistance and immune priming can evolve as mutually exclusive strategies, with distinct infection outcomes. However, the evolutionary drivers of such diverse immune functions remain poorly understood. Here, we addressed this key issue by systematically analyzing the differential fitness costs and benefits of priming vs. resistance evolution in Tribolium beetle populations infected with Bacillus thuringiensis . Surprisingly, resistant beetles had increased post-infection reproduction and a longer lifespan under both starving as well as fed conditions, with no other measurable costs. In contrast, priming reduced offspring early survival, development rate and reproduction. Priming did improve post-infection survival of offspring, but this added trans-generational benefit of immune priming might not compensate for its pervasive costs. Resistance was thus consistently more beneficial. Our work represents the first report of experimentally evolved trans-generational priming, and a detailed comparison of the complex fitness consequences of evolved priming vs resistance. HIGHLIGHTS Divergent costs and benefits of experimentally evolved immune priming vs resistance Increased reproduction and lifespan in resistant populations No other hidden costs of resistance In contrast, reduced juvenile fitness and reproduction in primed populations First evidence for experimentally evolved trans-generational immune priming